1. Field of the Invention
The present invention relates to a purge system for an internal combustion engine.
2. Description of Related Art
A purge system is a system that limits dispersion of fuel vapor into a surrounding environment upon generation, i.e., evaporation of the fuel vapor in a fuel tank. Specifically, the fuel vapor is supplied from the fuel tank into a canister, which contains an adsorbent to temporarily adsorb the fuel vapor. The fuel vapor, which is adsorbed by the adsorbent, is desorbed from the adsorbent with aid of a negative pressure that is generated in an intake air pipe during an operation of the engine. Thus, the desorbed fuel vapor is mixed with intake air to form a mixture gas, which is in turn purged into the intake air pipe through a purge passage.
Each of Japanese Unexamined Patent Publication No. H11-173220 (U.S. Pat. No. 6,138,644) and Japanese Unexamined Patent Publication No. H11-287162 recites a purge system of the internal combustion engine, which has a supercharging device in the intake air passage. In the purge system recited in Japanese Unexamined Patent Publication No. H11-173220, the purge of the fuel vapor is made possible even when the pressure in the intake air passage becomes a negative pressure due to a supercharging operation of the supercharging device. This is made possible by communicating the purge passage to an upstream side point of the intake air passage, which is on a upstream side of the supercharging device, and also placing a purge control valve and an electric pump in the purge passage. During the purge operation of the purge system, a constant drive electric current is supplied to the electric pump, and an opening degree of the purge control valve is controlled according to a required purge air quantity.
In the purge system recited in Japanese Unexamined Patent Publication No. H11-287162, a first purge valve is provided in a first purge passage, which communicates between the canister and a downstream side point of the intake air passage, which is located on a downstream side of the throttle valve. Also, a second purge valve is provided in a second purge passage, which communicates between the canister and the upstream side point of the intake air passage, which is located on the upstream side of the supercharging device. During the supercharging operation of the supercharging device, the second purge valve is opened to purge the fuel vapor from the second purge passage. During an absence of the supercharging operation of the supercharging device, the first purge valve is opened to purge the fuel vapor from the first purge passage.
However, in the case of the purge system recited in Japanese Unexamined Patent Publication No. H11-173220, when the electric pump is provided in the purge passage to purge the fuel vapor by the air flow created by the electric pump, an increase in the electric power consumption of the purge system, an increase in an installation space of the purge system and an increase in the costs disadvantageously occur.
In contrast, in the case of the purge system recited in Japanese Unexamined Patent Publication No. H11-287162, the negative pressure, which is created by the operation of the throttle valve or the supercharging device, is used to purge the fuel vapor without use of the electric pump. Thus, the above disadvantages will not occur in this purge system. However, the negative pressure, which is developed on the upstream side of the supercharging device through the operation of the supercharging device is generally small. Thus, it is often difficult to implement a sufficient purge quantity during the supercharging operation of the supercharging device.